Interferon-gamma and the induction of protective IgG2a antibodies in non-lethal Plasmodium berghei infections of mice

CD4 T Cell-Dependent and -Independent Roles for IFN-γ in Blood-Stage Malaria

Production of IFN-γ by CD4 T cells is widely theorized to control Plasmodium parasite burden during blood-stage malaria. Surprisingly, the specific and crucial mechanisms through which this highly pleiotropic cytokine acts to confer protection against malarial disease remain largely untested in vivo. Here we used a CD4 T cell-restricted Cre-Lox IFN-γ excision mouse model to test whether and how CD4 T cell-derived IFN-γ controls blood-stage malaria. Although complete absence of IFN-γ compromised control of the acute and the chronic, recrudescent blood-stage infections with P. c. chabaudi, we identified a specific, albeit modest, role for CD4 T cell-derived IFN-γ in limiting parasite burden only during the chronic stages of P. c. chabaudi malaria. CD4 T cell IFN-γ promoted IgG Ab class switching to the IgG2c isotype during P. c. chabaudi malaria in C57BL/6 mice. Unexpectedly, our data do not support gross defects in phagocytic activity in IFN-γ-deficient hosts infected with blood-stage malaria. Together, our data confirm CD4 T cell-dependent roles for IFN-γ but suggest CD4 T cell-independent roles for IFN-γ in immune responses to blood-stage malaria.

AFF3, a susceptibility factor for autoimmune diseases, is a molecular facilitator of immunoglobulin class switch recombination

Immunoglobulin class switch recombination (CSR) plays critical roles in controlling infections and inflammatory tissue injuries. Here, we show that AFF3, a candidate gene for both rheumatoid arthritis and type 1 diabetes, is a molecular facilitator of CSR with an isotype preference. Aff3-deficient mice exhibit low serum levels of immunoglobulins, predominantly immunoglobulin G2c (IgG2c) followed by IgG1 and IgG3 but not IgM. Furthermore, Aff3-deficient mice show weak resistance to Plasmodium yoelii infection, confirming that Aff3 modulates immunity to this pathogen. Mechanistically, the AFF3 protein binds to the IgM and IgG1 switch regions via a C-terminal domain, and Aff3 deficiency reduces the binding of AID to the switch regions less efficiently. One AFF3 risk allele for rheumatoid arthritis is associated with high mRNA expression of AFF3, IGHG2, and IGHA2 in human B cells. These findings demonstrate that AFF3 directly regulates CSR by facilitating the recruitment of AID to the switch regions.

Neddylation contributes to CD4+ T cell-mediated protective immunity against blood-stage Plasmodium infection

CD4⁺ T cells play predominant roles in protective immunity against blood-stage Plasmodium infection, both for IFN-γ-dependent effector mechanisms and providing B cell helper signals. Neddylation, an ubiquitination-like process triggered by covalent conjugation of NEDD8 to specific targets, has emerged as a potential regulator of T cell activities to TCR engagement. However, its contribution to T cell-mediated immunity to blood-stage malaria remains unclear. Here using an experimental model induced by Plasmodium yoelii 17XNL, and conditional knockout mice with T cell-specific deficiency of crucial components of neddylation pathway, we demonstrate activation of neddylation in T cells during blood-stage Plasmodium infection is essential for parasite control and host survival. Mechanistically, we show that apart from promoting CD4⁺ T cell activation, proliferation, and development of protective T helper 1 (Th1) cell response as suggested previously, neddylation is also required for supporting CD4⁺ T cell survival, mainly through B-cell lymphoma-2 (Bcl-2) mediated suppression of the mitochondria-dependent apoptosis. Furthermore, we provide evidence that neddylation contributes to follicular helper T (Tfh) cell differentiation, probably via augmenting the ubiquitin ligase Itch activity and proteasomal degradation of FoxO1, thereby facilitating germinal center (GC) formation and parasite-specific antibody production. This study identifies neddylation as a positive regulator of anti-Plasmodium immunity and provides insight into an involvement of such pathway in host resistance to infectious diseases.

Malaria, which is caused by the intracellular parasite Plasmodium, remains a major infectious disease with significant morbidity and mortality annually. Better understanding of the molecular mechanisms involved in protective immunity against the pathogenic blood-stage Plasmodium will facilitate development of anti-malarial drugs and vaccines. Neddylation has recently been identified as a potential regulator of T cell function. Here, we directly addressed the effects of neddylation on T cell responses and the outcome of blood-stage P. yoelii 17XNL malaria. We show that activation of neddylation in T cells is essential for IFN-γ-mediated proinflammatory response and generation of parasite-specific antibodies, thus contributing to full resolution of the infection. This is primarily associated with the reported beneficial effects of neddylation on CD4⁺ T cell activities, including activation, proliferation, and differentiation into T helper 1 (Th1) cells. Additionally, we establish a novel role of neddylation in parasite-responsive CD4⁺ T cell survival and follicular helper T (Tfh) cell differentiation. Therefore, we provide evidence that neddylation may represent a novel mechanism in orchestrating optimum CD4⁺ T cell effector response and subsequent humoral immunity to blood-stage Plasmodium infection.

Complement and Antibody-mediated Enhancement of Red Blood Cell Invasion and Growth of Malaria Parasites

Plasmodium falciparum malaria is a deadly pathogen. The invasion of red blood cells (RBCs) by merozoites is a target for vaccine development. Although anti-merozoite antibodies can block invasion in vitro, there is no efficacy in vivo. To explain this discrepancy we hypothesized that complement activation could enhance RBC invasion by binding to the complement receptor 1 (CR1). Here we show that a monoclonal antibody directed against the merozoite and human polyclonal IgG from merozoite vaccine recipients enhanced RBC invasion in a complement-dependent manner and that soluble CR1 inhibited this enhancement. Sialic acid-independent strains, that presumably are able to bind to CR1 via a native ligand, showed less complement-dependent enhancement of RBC invasion than sialic acid-dependent strains that do not utilize native CR1 ligands. Confocal fluorescent microscopy revealed that complement-dependent invasion resulted in aggregation of CR1 at the RBC surface in contact with the merozoite. Finally, total anti-P. berghei IgG enhanced parasite growth and C3 deficiency decreased parasite growth in mice. These results demonstrate, contrary to current views, that complement activation in conjunction with antibodies can paradoxically aid parasites invade RBCs and should be considered in future design and testing of merozoite vaccines.

A transient resistance to blood-stage malaria in interferon-γ-deficient mice through impaired production of the host cells preferred by malaria parasites

IFN-γ plays both pathological and protective roles during blood-stage malaria. One of its pathological roles is its contribution to anemia by suppressing erythropoiesis. Here, to evaluate the effects of IFN-γ-mediated alterations in erythropoiesis on the course of malaria infection, mice deficient in IFN-γ (GKO) were infected with two strains of the rodent malaria parasite Plasmodium yoelii, 17XL (PyL) and 17XNL (PyNL), whose host cell ranges differ. Regardless of genotype, all mice infected with PyL, which can invade any erythrocyte, developed high parasitemia and died quickly. Although PyNL caused a transient non-lethal infection in wild-type (WT) mice, some GKO mice were unable to control the infection and died. However, GKO mice were resistant to the early phase of infection, showing an impaired increase in parasitemia compared with WT mice. This resistance in the GKO mice was associated with having significantly fewer reticulocytes, which are the preferred host cells for PyNL parasites, than the WT mice. Compared with the amount of reticulocytes in GKO mice during the early stages of infection, there was a significant increase in the amount of these cells at later stages, which coincided with the inability of these mice to control the infection. We found that the growth of PyNL parasites correlated with the amount of reticulocytes. Thus, the reduced number of reticulocytes in mice lacking IFN-γ appears to be responsible for the limited parasite growth. Notably, these differences in GKO mice were at least partially reversed when the mice were injected with exogenous IFN-γ. Additionally, an artificial induction of hemolytic anemia and an increase in reticulocytes by phenylhydrazine treatment in GKO mice completely abolished the lower parasitemia and resistance during early phase infection. These results suggest that IFN-γ may contribute to the early growth of PyNL parasites by increasing the amount of reticulocytes, presumably by enhancing erythropoiesis.

Protective immunity against malaria after vaccination

A good understanding of the immunological correlates of protective immunity is an important requirement for the development of effective vaccines against malaria. However, this concern has received little attention even in the face of two decades of intensive vaccine research. Here, we review the immune response to blood-stage malaria, with a particular focus on the type of vaccine most likely to induce the kind of response required to give strong protection against infection.

Assessment of antibody-dependent respiratory burst activity from mouse neutrophils on Plasmodium yoelii malaria challenge outcome

New tools are required to expedite the development of an effective vaccine against the blood-stage infection with the human malaria parasite Plasmodium falciparum. This work describes the assessment of the ADRB assay in a mouse model, characterizing the functional interaction between antimalarial serum antibodies and FcRs upon neutrophils. We describe a reproducible, antigen-specific assay, dependent on functional FcR signaling, and show that ADRB activity is induced equally by IgG1 and IgG2a isotypes and is modulated by blocking FcR function. However, following immunization of mice with the blood-stage vaccine candidate antigen MSP142, no measurable ADRB activity was induced against PEMS and neither was vaccine efficacy modulated against Plasmodium yoelii blood-stage challenge in γ(-/-) mice compared with WT mice. In contrast, following a primary, nonlethal P. yoelii parasite challenge, serum from vaccinated mice and nonimmunized controls showed anti-PEMS ADRB activity. Upon secondary challenge, nonimmunized γ(-/-) mice showed a reduced ability to control blood-stage parasitemia compared with immunized γ(-/-) mice; however, WT mice, depleted of their neutrophils, did not lose their ability to control infection. Thus, whereas neutrophil-induced ADRB against PEMS does not appear to play a role in protection against P. yoelii rodent malaria, induction of ADRB activity after challenge suggests that antigen targets of anti-PEMS ADRB activity remain to be established, as well as further supporting the observation that ADRB activity to P. falciparum arises following repeated natural exposure.

The utility of Plasmodium berghei as a rodent model for anti-merozoite malaria vaccine assessment

Rodent malaria species Plasmodium yoelii and P. chabaudi have been widely used to validate vaccine approaches targeting blood-stage merozoite antigens. However, increasing data suggest the P. berghei rodent malaria may be able to circumvent vaccine-induced anti-merozoite responses. Here we confirm a failure to protect against P. berghei, despite successful antibody induction against leading merozoite antigens using protein-in-adjuvant or viral vectored vaccine delivery. No subunit vaccine approach showed efficacy in mice following immunization and challenge with the wild-type P. berghei strains ANKA or NK65, or against a chimeric parasite line encoding a merozoite antigen from P. falciparum. Protection was not improved in knockout mice lacking the inhibitory Fc receptor CD32b, nor against a Δsmac P. berghei parasite line with a non-sequestering phenotype. An improved understanding of the mechanisms responsible for protection, or failure of protection, against P. berghei merozoites could guide the development of an efficacious vaccine against P. falciparum.

mRNA expression of cytokines and its impact on outcomes after infection with lethal and nonlethal Plasmodium vinckei parasites

Cytokines play an important role in the defense against malaria and some have long been documented to influence the course of malaria infection in rodents and humans. The present study was conducted to determine the mRNA expression pattern of a few prominent cytokines at different time points during the course of infection with a nonlethal and lethal Plasmodium vinckei rodent malaria parasite, using highly sensitive real-time PCR. Analysis of mRNA expression of cytokines in spleen from infected mice revealed that the principal difference was an early depletion in pro-inflammatory cytokine's mRNA expression in mice infected with lethal P. vinckei (PvAS) parasites. In addition, an increase in anti-inflammatory cytokines particularly IL-10 mRNA expression levels was found in the same group of mice. In contrast, the significant rise in pro-inflammatory cytokine's mRNA expression levels was recorded at day 1 onwards after infection with nonlethal P. vinckei (PvAR). The maximum fold change was recorded for IFN-γ and IL-10, when compared to baseline value. TGF-β did not seem to play any major role in P. vinckei infection.

Experimental asexual blood stage malaria immunity

Immunity to asexual blood stages of malaria is complex, involving both humoral and cell-mediated immune mechanisms. The availability of murine models of malaria has greatly facilitated the analysis of immune mechanisms involved in resistance to the asexual blood stages. This unit details the materials and methods required for inducing protective immunity toward experimental blood stage malaria parasites by vaccination, repeated infection, and drug cure, as well as adoptive transfer of antigen-specific T cells.

Immune-mediated mechanisms of parasite tissue sequestration during experimental cerebral malaria

Cerebral malaria is a severe complication of malaria. Sequestration of parasitized RBCs in brain microvasculature is associated with disease pathogenesis, but our understanding of this process is incomplete. In this study, we examined parasite tissue sequestration in an experimental model of cerebral malaria (ECM). We show that a rapid increase in parasite biomass is strongly associated with the induction of ECM, mediated by IFN-gamma and lymphotoxin alpha, whereas TNF and IL-10 limit this process. Crucially, we discovered that host CD4(+) and CD8(+) T cells promote parasite accumulation in vital organs, including the brain. Modulation of CD4(+) T cell responses by helminth coinfection amplified CD4(+) T cell-mediated parasite sequestration, whereas vaccination could generate CD4(+) T cells that reduced parasite biomass and prevented ECM. These findings provide novel insights into immune-mediated mechanisms of ECM pathogenesis and highlight the potential of T cells to both prevent and promote infectious diseases.

Pretreatment with Cry1Ac protoxin modulates the immune response, and increases the survival of Plasmodium-infected CBA/Ca mice

Malaria is a major global health problem that kills 1-2 million people each year. Despite exhaustive research, naturally acquired immunity is poorly understood. Cry1A proteins are potent immunogens with adjuvant properties and are able to induce strong cellular and humoral responses. In fact, it has been shown that administration of Cry1Ac protoxin alone or with amoebic lysates induces protection against the lethal infection caused by the protozoa Naegleria fowleri. In this work, we studied whether Cry1Ac is able to activate the innate immune response to induce protection against Plasmodium berghei ANKA (lethal) and P. chabaudi AS (nonlethal) parasites in CBA/Ca mice. Treatment with Cry1Ac induced protection against both Plasmodium species in terms of reduced parasitaemia, longer survival time, modulation of pro- and anti-inflammatory cytokines, and increased levels of specific antibodies against Plasmodium. Understanding how to boost innate immunity to Plasmodium infection should lead to immunologically based intervention strategies.

Functional memory B cells and long-lived plasma cells are generated after a single Plasmodium chabaudi infection in mice

Antibodies have long been shown to play a critical role in naturally acquired immunity to malaria, but it has been suggested that Plasmodium-specific antibodies in humans may not be long lived. The cellular mechanisms underlying B cell and antibody responses are difficult to study in human infections; therefore, we have investigated the kinetics, duration and characteristics of the Plasmodium-specific memory B cell response in an infection of P. chabaudi in mice. Memory B cells and plasma cells specific for the C-terminal region of Merozoite Surface Protein 1 were detectable for more than eight months following primary infection. Furthermore, a classical memory response comprised predominantly of the T-cell dependent isotypes IgG2c, IgG2b and IgG1 was elicited upon rechallenge with the homologous parasite, confirming the generation of functional memory B cells. Using cyclophosphamide treatment to discriminate between long-lived and short-lived plasma cells, we demonstrated long-lived cells secreting Plasmodium-specific IgG in both bone marrow and in spleens of infected mice. The presence of these long-lived cells was independent of the presence of chronic infection, as removal of parasites with anti-malarial drugs had no impact on their numbers. Thus, in this model of malaria, both functional Plasmodium-specific memory B cells and long-lived plasma cells can be generated, suggesting that defects in generating these cell populations may not be the reason for generating short-lived antibody responses.

Malaria causes considerable human suffering resulting from associated high mortality, morbidity and reduced economic productivity in endemic areas. Current control methods are thwarted by a multiplicity of problems including rapidly developing resistance for anti-malarial drugs and insecticide-treated nets, and huge costs and hence poor coverage with bed nets in poor countries. Understanding the basis of the inefficiency of immunity to malaria in childhood will greatly aid the search for effective vaccines, which together with drugs and vector control, will be essential in the drive to eliminate malaria. Because of the strong evidence associating anti-malarial antibodies with anti-parasitic and anti-disease effects, vaccines inducing protective long-lasting antibody responses are attractive. However, it has been suggested that antibody responses to some Plasmodium antigens may be not long-lived. It would be important to determine whether long-lived plasma cells and memory B cells are generated after a malaria infection; however, these studies are difficult to perform in humans. Therefore we investigated the kinetics, duration and characteristics of the two cell types responsible for long-term antibody production in a mouse model of malaria. We show here that malaria-specific memory B cells and plasma cells are still detectable more than eight months after infection, and that both long-lived malaria-specific antibody-secreting cells and functional malaria-specific memory B cells can be made after a single infection.

Specific anti-Neospora caninum IgG1 and IgG2 antibody responses during gestation in naturally infected cattle and their relationship with gamma interferon production

Gamma interferon (IFN-gamma) production has been attributed a protective role against abortion in Neospora-infected dairy cows. This study investigate possible relationships between plasma levels of the N. caninum-specific immunoglobulin (Ig) isotypes IgG1 and IgG2 and IFN-gamma production throughout gestation in dairy cows naturally infected with the parasite. Data were obtained from 31 pregnant Neospora-seropositive animals. Blood samples were collected on gestation days 40, 90, 120, 150, 180 and 210 or until abortion. Ten pregnancies ended in abortion. One of the 11 cows producing IFN-gamma suffered abortion (9.1% of the cows producing IFN-gamma). Non-aborting cows producing IFN-gamma (n=10) showed a significantly higher IgG2/IgG1 antibody ratio throughout the gestation period than cows not producing IFN-gamma (n=11). A significant negative effect of IFN-gamma production on IgG1 antibodies was also observed, while IFN-gamma production did not affect IgG2 antibody levels. In contrast, higher levels of IgG2 antibodies compared to IgG1 antibodies were observed over the whole of gestation in aborting animals, both in those not producing IFN-gamma and in the single aborting cow that produced the cytokine. Our findings indicate that a Th1 immune response, in which IgG2 antibodies prevail, could be protective against N. caninum abortion, but only in the presence of IFN-gamma production. On their own, elevated IgG2 antibody titres appear to be insufficient to protect dams chronically infected with N. caninum against abortion.

Plasmodium berghei XAT: contribution of gammadelta T cells to host defense against infection with blood-stage nonlethal malaria parasite

We examined a potential role of gammadelta T cells in protective immunity to blood-stage Plasmodium berghei XAT infection. Plasmodium berghei XAT is an attenuated variant of the lethal strain P. berghei NK65 and its infection is self-resolving in immune competent mice. To determine whether gammadelta T cells are essential for the resolution of P. berghei XAT malaria, mice were depleted of gammadelta T cells with anti-TCRgammadelta antibody treatment. Although mice that had received control antibody resolved infections, mice received anti-TCRgammadelta antibody could not control their infections and eventually died. Spleen cells from infected mice produced IFN-gamma and nitric oxide (NO) within the first week of infection, however, levels of IFN-gamma and NO in gammadelta T cell-depleted mice were significantly lower than in control mice. To examine whether gammadelta T cells are involved in the antibody production, malarial-specific antibodies of the various isotypes were measured in the sera of gammadelta T cell-depleted mice and control mice. Serum levels of IgG2a, which was known to be a protective antibody in P. berghei XAT malaria, were significantly lower in gammadelta T cell-depleted mice than in control mice, whereas levels of IgG1 were comparable to those in control mice. Our results indicated that the presence of the gammadelta T cell subset was essential for resolution of blood-stage P. berghei XAT malaria and played a modulatory role in the development of Th1 response and host defense against this malarial parasites.

Long-lasting protective immune response to the 19-kilodalton carboxy-terminal fragment of Plasmodium yoelii merozoite surface protein 1 in mice

Merozoite surface protein 1 (MSP1) is the major protein on the surface of the plasmodial merozoite, and its carboxy terminus, the 19-kDa fragment (MSP1(19)), is highly conserved and effective in induction of a protective immune response against malaria parasite infection in mice and monkeys. However, the duration of the immune response has not been elucidated. As such, we immunized BALB/c mice with a standard four-dose injection of recombinant Plasmodium yoelii MSP1(19) formulated with Montanide ISA51 and CpG oligodeoxynucleotide (ODN) and monitored the MSP1(19)-specific antibody levels for up to 12 months. The antibody titers persisted constantly over the period of time without significant waning, in contrast to the antibody levels induced by immunization with Freund's adjuvant, where the antibody levels gradually declined to significantly lower levels 12 months after immunization. Investigation of immunoglobulin G (IgG) subclass longevity revealed that only the IgG1 antibody level (Th2 type-driven response) decreased significantly by 6 months, while the IgG2a antibody level (Th1 type-driven response) did not change over the 12 months after immunization, but the boosting effect was seen in the IgG1 antibody responses but not in the IgG2a antibody responses. After challenge infection, all immunized mice survived with negligibly patent parasitemia. These findings suggest that protective immune responses to MSP1(19) following immunization using oil-based Montanide ISA51 and CpG ODN as an adjuvant are very long-lasting and encourage clinical trials for malaria vaccine development.

Differential antibody responses to Plasmodium falciparum-derived B-cell epitopes induced by diepitope multiple antigen peptides (MAP) containing different T-cell epitopes

Epitopes of universal character are needed when designing subunit vaccines against infectious diseases such as malaria. We have compared the immunogenicity of B-cell epitopes from the Plasmodium falciparum antigen repeats DPNANPNV (PfCS protein) and VTEEI (Pf332) when assembled with four different universal T-cell epitopes in diepitope multiple antigen peptides (MAP). T-epitopes employed were from P. falciparum antigens (CS.T3, [T(*)]4 and EBP3) or from the Clostridium tetani toxin (P2). In association with either of the T-epitopes, the genetic unresponsiveness to the B-epitopes was successfully bypassed. Our results show that the immunogenicity of a T-epitope alone does not necessarily predict the ability of the T-epitope to provide T-cell help when combined with other epitopes in an immunogen. Further, the nature of the immune responses in terms of total IgG antibodies and their subclass distribution, T-cell proliferation and IFN-gamma production, varied with the T-epitope and mouse strain, which may indicate the need for inclusion of a combination of different universal T-epitopes in a future malaria subunit vaccine.

Early treatment during a primary malaria infection modifies the development of cross immunity

We have used a murine model to study the kinetics of cross-protection when a primary infection is halted at different times. We analysed how parasitaemia is modified during a second infection with the homologous parasite, a heterologous parasite, or a mixture of the two. In addition, possible mechanisms involved in cross-protection were analysed. Results show that treatment with pyrimethamine on day 5 during a primary infection with P. chabaudi AS (non-lethal), prevents the generation of cross-protection to a new challenge with lethal P. yoelii 17XL. In contrast, when treatment is on day 7, mice survive a P. yoelii infection. Differences between both groups suggest that in order for 'preimmune' mice to survive a lethal challenge, a predominantly TH2-type response is required, with a higher mRNA expression level of IL-4 and IL-10, and a lower mRNA expression of IFN-gamma. This work shows that an early treatment of a malaria infection produced by a non-lethal parasite drives the immune response towards a loss of cross-protection to further infections, in particular with more virulent parasites. This finding should be taken into account for the development of effective malaria vaccines.

Induction of IgG2a class switching in B cells by IL-27

IL-27 is a novel IL-12 family member that plays a role in the early regulation of Th1 initiation. However, its role in B cells remains unexplored. We here show a role for IL-27 in the induction of T-bet expression and regulation of Ig class switching in B cells. Expression of WSX-1, one subunit of IL-27R, was detected at the mRNA level in primary mouse spleen B cells, and stimulation of these B cells by IL-27 rapidly activated STAT1. IL-27 then induced T-bet expression and IgG2a, but not IgG1, class switching in B cells activated with anti-CD40 or LPS. In contrast, IL-27 inhibited IgG1 class switching induced by IL-4 in activated B cells. Similar induction of STAT1 activation, T-bet expression and IgG2a class switching was observed in IFN-gamma-deficient B cells, but not in STAT1-deficient ones. The induction of IgG2a class switching was abolished in T-bet-deficient B cells activated with LPS. These results suggest that primary spleen B cells express functional IL-27R and that the stimulation of these B cells by IL-27 induces T-bet expression and IgG2a, but not IgG1, class switching in a STAT1-dependent but IFN-gamma-independent manner. The IL-27-induced IgG2a class switching is highly dependent on T-bet in response to T-independent stimuli such as LPS. Thus, IL-27 may be a novel attractive candidate as a therapeutic agent against diseases such as allergic disorders by not only regulating Th1 differentiation but also directly acting on B cells and inducing IgG2a class switching.

Association between sequence polymorphism in an epitope of Babesia divergens Bd37 exoantigen and protection induced by passive transfer

In Europe, Babesia divergens is the major agent responsible for babesiosis in cattle and can occasionally infect splenectomised humans. Recently, we reported the characterisation of a 37 kDa exoantigen (Bd37) anchored in the merozoite membrane of B. divergens by a glycosylphosphatidyl-inositol. After phospholipase hydrolyse of the glycosylphosphatidyl-inositol anchor, the Bd37 antigen could be isolated in the plasma of the infected host and from the in vitro culture supernatants. Immunisation of mice with a gel-filtration protective fraction of B. divergens exoantigens, produced a monoclonal antibody (MAb), called F4.2F8-INT, directed against Bd37. In the present study, we report data on passive protection using MAb F4.2F8-INT. This MAb was able to completely protect against virulent challenges with B. divergens isolates Rouen 1987 (Rouen87) and Weybridge 8843 (W8843) but had no protective effect against another French isolate from Massif Central (6303E). Physical characterisation of the epitope recognised by F4.2F8-INT allowed us to explain the differences observed between these isolates by western blotting and passive protection. These results suggest that the antigen carrying this epitope could be used as a target in the development of a recombinant vaccine against B. divergens babesiosis.

Gene gun-based co-immunization of merozoite surface protein-1 cDNA with IL-12 expression plasmid confers protection against lethal Plasmodium yoelii in A/J mice

The carboxyl-terminal region of the merozoite surface protein-1 (MSP1) is a leading candidate for a vaccine against malaria in the erythrocytic stage. In this study, we investigated the utility of interleukin-12 (IL-12) cDNA as an adjuvant for malaria DNA vaccine in a mouse challenge model. We found that co-immunization of expression plasmids encoding a C-terminal 15-kDa fragment of MSP1 (MSP1-15) with the IL-12 gene using a gene gun significantly increased the protective immunity against malaria as compared with MSP1-15 DNA immunization alone. Co-immunization of IL-12 DNA potentiated MSP1-15-specific T helper (Th)1-type immune responses as evaluated by in vivo antibody (Ab) responses and in vitro cytokine profiles. After the Plasmodium yoelii challenge, mice immunized with MSP1-15 plus IL-12 DNA showed a higher level of interferon gamma (IFN-gamma) production than did other groups of mice. In vivo neutralization of IFN-gamma or depletion of CD4(+) T cells completely abolished this protective immunity. Macrophages, but not nitric oxide (NO), were found to play an important role in this effector mechanism. The sera from mice in which the infection had been cleared by the vaccination showed strong protection against P. yoelii infection. Thus, in addition to cellular immune responses, Abs against parasites induced in the course of infection are essential for protection against P. yoelii. The results indicate that combined vaccination with DNA encoding antigenic peptides plus IL-12 DNA provides a strategy for improving the prophylactic efficacy of a vaccine for malaria infection.

IL-12 is required for antibody-mediated protective immunity against blood-stage Plasmodium chabaudi AS malaria infection in mice

In this study, we investigated the role of endogenous IL-12 in protective immunity against blood-stage P. chabaudi AS malaria using IL-12 p40 gene knockout (KO) and wild-type (WT) C57BL/6 mice. Following infection, KO mice developed significantly higher levels of primary parasitemia than WT mice and were unable to rapidly resolve primary infection and control challenge infection. Infected KO mice had severely impaired IFN-gamma production in vivo and in vitro by NK cells and splenocytes compared with WT mice. Production of TNF-alpha and IL-4 was not compromised in infected KO mice. KO mice produced significantly lower levels of Th1-dependent IgG2a and IgG3 but a higher level of Th2-dependent IgG1 than WT mice during primary and challenge infections. Treatment of KO mice with murine rIL-12 during the early stage of primary infection corrected the altered IgG2a, IgG3, and IgG1 responses and restored the ability to rapidly resolve primary and control challenge infections. Transfer of immune serum from WT mice to P. chabaudi AS-infected susceptible A/J mice completely protected the recipients, whereas immune serum from KO mice did not, as evidenced by high levels of parasitemia and 100% mortality in recipient mice. Furthermore, depletion of IgG2a from WT immune serum significantly reduced the protective effect of the serum while IgG1 depletion had no significant effect. Taken together, these results demonstrate the protective role of a Th1-immune response during both acute and chronic phases of blood-stage malaria and extend the immunoregulatory role of IL-12 to Ab-mediated immunity against Plasmodium parasites.

Susceptibility of the different developmental stages of the asexual (schizogonic) erythrocyte cycle of Plasmodium chabaudi chabaudi to hyperimmune serum, immunoglobulin (Ig)G1, IgG2a and F(ab')2 fragments

The mechanisms by which antibodies interfere with Plasmodium growth are still under debate. Characterizing the asexual erythrocyte stages susceptible to antibodies from hyperimmune individuals is therefore a relevant contribution to vaccine research. In this study, using a virulent and synchronous murine malaria parasite, Plasmodium chabaudi chabaudi AJ, we have shown that trophozoites and circulating schizonts are not the main targets for antibodies from hyperimmune serum. In drug-cured mice challenged with a high inoculum of ring-infected erythrocytes, parasitemias do not decline until the moment of erythrocyte rupture, suggesting that effector mechanisms operate immediately prior to reinvasion. Confirming these findings, treatment of primary-infected mice with hyperimmune serum inhibited the generation of new ring forms, but did not alter the numbers of schizont-infected erythrocytes, despite the fact that these cells were recognized by immunoglobulin (Ig)G antibodies. When these mice were treated with IgG1 or IgG2a purified from hyperimmune serum, both subclasses limited reinvasion, but IgG2a showed a stronger protective activity. The fact that Fc digestion decreases but does not abrogate protection suggests that both Fc-dependent and independent mechanisms participate in this process. Treatment with cobra venom factor did not interfere with the antibody-mediated protection, ruling out the participation of the complement system in both lysis and phagocytosis of merozoites or infected erythrocytes. Therefore, in mice suffering from P. c. chabaudi AJ malaria, merozoite neutralization seems to be a major mechanism of protection conferred by hyperimmune serum antibodies. However, FcgammaR-mediated interactions, or other mechanisms not yet defined, may also contribute to inhibit erythrocyte reinvasion.

A critical role of Fc receptor-mediated antibody-dependent phagocytosis in the host resistance to blood-stage Plasmodium berghei XAT infection

Plasmodium berghei XAT is an irradiation-induced attenuated variant derived from the lethal strain P. berghei NK65, and its blood-stage parasites are spontaneously cleared in immune competent mice. In the present study, we studied the mechanism of host resistance to blood-stage malaria infection using P. berghei XAT. Infection enhanced Ab-dependent phagocytosis of PRBC by splenic macrophages in wild-type C57BL/6 mice. In contrast, FcR gamma-chain knockout (FcRgamma(-/-)) mice, which lack the ability to mediate Ab-dependent phagocytosis and Ab-dependent cell-mediated cytotoxicity through FcgammaRI, FcgammaRII, and FcgammaRIII, could not induce Ab-dependent phagocytic activity. These FcRgamma(-/-) mice showed increased susceptibility to the P. berghei XAT infection, with eventually fatal results, although they produced comparable amounts of IFN-gamma by spleen cells and anti-XAT Abs in serum. In addition, passive transfer of anti-XAT IgG obtained from wild-type mice that had recovered from infection into FcRgamma(-/-) mice could not suppress the increase in parasitemia, and almost all of these mice died after marked parasitemia. In contrast, passive transfer of anti-XAT IgG into control wild-type mice inhibited the increase in parasitemia. IFN-gamma(-/-) mice, which were highly susceptible to the P. berghei XAT infection, failed to induce Ab-dependent phagocytic activity and also showed reduced production of serum anti-XAT IgG2a isotype compared with control wild-type mice. These results suggest that FcR-mediated Ab-dependent phagocytosis, which is located downstream of IFN-gamma production, is important as an effector mechanism to eliminate PRBC in blood-stage P. berghei XAT infection.

Mycobacterium bovis bacillus calmette-guérin induces protective immunity against infection by Plasmodium yoelii at blood-stage depending on shifting immunity toward Th1 type and inducing protective IgG2a after the parasite infection

Bacillus calmette-guérin (BCG)-vaccination raised dramatically the survival rates of A/J mice from infection by Plasmodium yoelii 17XL at blood-stage. The analysis of the immune response of spleen cells indicated that BCG vaccination biased the immune response toward Th1 type. Neutralization of IFN-gamma and nitric oxide abrogated the protection. The kinetics of Ab production in the course of P. yoelii 17XL infection was monitored. Surprisingly, larger amounts of parasite-specific Abs were produced in BCG-vaccinated mice than in the placebo control. The vast majority of the produced IgG against parasites in BCG-vaccinated mice was IgG2a, which was observed hardly in placebo controls. The peak of IgG2a production coincided with the clearance of infection. The naive mice transferred adoptively with IgG2a from self-cured mice survived the lethal challenge from the parasite. These data indicated that BCG vaccination protected A/J mouse from P. yoelii 17XL infection by biasing immunity toward Th1-type after parasite infection and enhancing production of IgG2a, which ultimately played a major role in protection.

Helper T cell and antibody responses to infection of CBA mice with Babesia microti

Helper T cell cytokine and antibody responses were investigated in mice after infection with Babesia microti (King strain). Infection of CBA mice with 106 parasitized erythrocytes resulted in the development of a transitory high parasitaemia which peaked 14 days post infection (DPI), and was resolved at 24 DPI. Th1 responses were activated predominately during the acute phase (6-18 DPI) whereas Th2 responses predominated during the recovery phase (14-28 DPI) as detected by the reverse transcriptase polymerase chain reaction. Increased expression of Th1 cytokines was first detected at 6 DPI (IL-2) and 8 DPI (IFN-gamma) and their peak levels were reached at 12 DPI. After the peak levels were reached, they progressively declined and fell to baseline levels (22 DPI). Increased expression of Th2 cytokines (IL-4 and IL-10) first appeared at 14 DPI, peaked at 20 DPI and Th2 cytokine levels were elevated till the end of the study (28 DPI). Levels of serum IFN-gamma detected by a sandwich ELISA correlated well with IFN-gamma gene expression and were detectable at 8-18 DPI. IgM against B. microti was first detected in serum by ELISA at 4 DPI, and peaked at 10 DPI. The levels of IgM subsequently declined but remained positive at low titre till the end of study. IgG against B. microti was first detected at 8 DPI and peak levels were reached at 24 DPI and remained at that level until the end of study. The results of the present study show that Th1 cytokines predominated in the early inflammatory response and might be involved in control of levels of acute parasitaemia whereas the Th2-associated responses, including expression of IL-4 and IL-10 and the production of parasite-specific IgG, might be the functional means for the reduction and clearance of the parasite from the body. It was concluded that an effective vaccine against Babesia spp. should be designed to induce Th1 responses to maintain the parasitaemia at unfulminating levels and also maintain Th2 responses to clear the parasite from the body.

IFN-gamma-independent IgG2a production in mice infected with viruses and parasites

After infection with some viruses and intracellular parasites, antibody production is restricted to IgG2a. We first observed that, whereas live viruses such as lactate dehydrogenase-elevating virus (LDV) or mouse adenovirus induced mostly an IgG2a response, a large proportion of antibodies produced against killed viruses were IgG1. This IgG1 antiviral response was suppressed when live virions were added to inactivated viral particles. These results indicate that the IgG2a preponderance is related to the infectious process itself rather than to the type of antigen involved. Since IFN-gamma is known to stimulate IgG2a production by activated B lymphocytes and to be secreted after infection, we examined the role of this cytokine in the antibody isotypic distribution caused by LDV. Most IgG2a responses were relatively unaffected in mice deficient for the IFN-gamma receptor or treated with anti-IFN-gamma antibody. A similar IFN-gamma-independent IgG2a secretion was observed after infection with the parasites Toxoplasma gondii and Trypanosoma cruzi. However, the IFN-gamma-independent IgG2a production triggered by infection still required the presence of functional T(h) lymphocytes. Therefore, signal(s) other than IFN-gamma secretion may explain the T(h)-dependent isotypic bias in antibody secretion triggered by viruses and parasites.

Correlation of T-cell response and lymphokine profile with RESA peptides of Plasmodium falciparum containing a universal T-cell epitope and an immunopotentiator, polytuftsin

Two RESA repeat sequences, (EENVEHDA)2 and (DDEHVEEPTVA)2, were chemically linked to a universal T-cell epitope, CS.T3 and polytuftsin, and a natural immunopotentiator, was physically mixed with these conjugates. The immunogens were studied for in vitro antigen-induced T-cell proliferation, and cytokine levels were measured in the culture supernatants. The RESA peptide(s)-CS.T3 conjugate containing polytuftsin showed the highest stimulation index (SI) as compared to the RESA peptide-CS.T3 conjugates or RESA peptides alone. Spleen cells from mice primed with either RESA peptide(s)-CS.T3 conjugate or RESA peptide-CS.T3 conjugate containing polytuftsin, when pulsed in vitro with the respective RESA peptide, showed a higher proliferation index as compared to spleen cells primed and pulsed in vitro with the respective RESA peptides. This observation has an important relevance during natural reinfection for boosting the immune response. The culture supernatants from the cells primed and pulsed in vitro with RESA peptide-CS.T3 conjugate and RESA peptide-CS.T3 conjugate containing polytuftsin showed higher IL-2 and IFN-gamma levels as compared to the RESA peptides alone. Very low IL-4 levels were detected with the above formulations. The cytokine profile is suggestive of a CD4+ TH1 type of immune response, which is ideal for the killing of intracellular pathogens like the malarial parasite.

Gamma interferon production is critical for protective immunity to infection with blood-stage Plasmodium berghei XAT but neither NO production nor NK cell activation is critical

We have examined the roles of gamma interferon (IFN-gamma), nitric oxide (NO), and natural killer (NK) cells in the host resistance to infection with the blood-stage malarial parasite Plasmodium berghei XAT, an irradiation-induced attenuated variant of the lethal strain P. berghei NK65. Although the infection with P. berghei XAT enhanced NK cell lytic activity of splenocytes, depletion of NK1.1(+) cells caused by the treatment of mice with anti-NK1.1 antibody affected neither parasitemia nor IFN-gamma production by their splenocytes. The P. berghei XAT infection induced a large amount of NO production by splenocytes during the first peak of parasitemia, while P. berghei NK65 infection induced a small amount. Unexpectedly, however, mice deficient in inducible nitric oxide synthase (iNOS-/-) cleared P. berghei XAT after two peaks of parasitemia were observed, as occurred for wild-type control mice. Although the infected iNOS-/- mouse splenocytes did not produce a detectable level of NO, they produced an amount of IFN-gamma comparable to that produced by wild-type control mouse splenocytes, and treatment of these mice with neutralizing anti-IFN-gamma antibody led to the progression of parasitemia and fatal outcome. CD4(-/-) mice infected with P. berghei XAT could not clear the parasite, and all these mice died with apparently reduced IFN-gamma production. Furthermore, treatment with carrageenan increased the susceptibility of mice to P. berghei XAT infection. These results suggest that neither NO production nor NK cell activation is critical for the resistance to P. berghei XAT infection and that IFN-gamma plays an important role in the elimination of malarial parasites, possibly by the enhancement of phagocytic activity of macrophages.

Plasmodium chabaudi chabaudi: effect of low parasitemias on immunity in CB6F1 mice

We examined the effect that low parasitemias have on the immune response of CB6F1 mice infected with Plasmodium chabaudi chabaudi AS. Ascending parasitemias were stopped by chloroquine treatment when they were between 1.6 and 9.4%. Mice that suffered low parasitemias developed good immunity to homologous reinfection but, contrary to what happened in mice that suffered full parasitemias, they did not develop immunity to heterologous reinfection with Plasmodium yoelii 17XL. Total IgG antiparasite antibody responses were similar in mice that suffered low or full parasitemia, both in primary infection and after reinfection. At the level of isotypes, IgM, IgG1, IgG2b, and IgG3 responses were similar in mice that suffered low or full parasitemias, but after reinfection, mice that suffered low parasitemias responded with higher levels of IgG2a than mice that suffered full parasitemias. Mice that suffered low parasitemias did not have splenomegaly but their immunity to homologous reinfection was diminished after splenectomy in a manner similar to that of splenectomized mice that suffered full parasitemia. CB6F1 mice can develop homologous immunity even if exposed to low parasitemias but cannot develop heterologous immunity unless exposed to high parasite loads.

Cancer Dormancy. VII. A Regulatory Role for CD8+ T Cells and IFN-γ in Establishing and Maintaining the Tumor-Dormant State

Dormant tumor cells resistant to ablative cancer therapy represent a significant clinical obstacle due to later relapse. Experimentally, the murine B cell lymphoma (BCL1) is used as a model of tumor dormancy in mice vaccinated with the BCL1 Ig. Here, we used this model to explore the cellular mechanisms underlying dormancy. Our previous studies have demonstrated that T cell-mediated immunity is an important component in the regulation of tumor dormancy because Id-immune T cells adoptively transferred into passively immunized SCID mice challenged with BCL1 cells significantly increased the incidence and duration of the dormant state. We have extended these observations and demonstrate that CD8+, but not CD4+, T cells are required for the maintenance of dormancy in BCL1 Ig-immunized BALB/c mice. In parallel studies, the transfer of Id-immune CD8+ cells, but not Id-immune CD4+ cells, conferred significant protection to SCID mice passively immunized with nonprotective levels of polyclonal anti-Id and then challenged with BCL1 cells. Furthermore, the ability of CD8+ T cells to induce a state of dormancy in passively immunized SCID mice was completely abrogated by treatment with neutralizing α-IFN-γ mAbs in vivo. In vitro studies demonstrated that IFN-γ alone or in combination with reagents to cross-link the surface Ig induced both cell cycle arrest and apoptosis in a BCL1 cell line. Collectively, these data demonstrate a role for CD8+ T cells via endogenous production of IFN-γ in collaboration with humoral immunity to both induce and maintain a state of tumor dormancy.

Chronic rejection of major histocompatibility complex class II-disparate skin grafts after anti-CD3 therapy: a model of antibody-independent transplant vasculopathy

BACKGROUND

Chronic rejection remains a leading cause of allograft loss. Histologically, it is characterized by arterial intimal thickening and parenchymal fibrosis. The immune mechanisms triggering chronic rejection are still uncompletely understood.

METHODS

We performed major histocompatibility complex (MHC) class H-incompatible skin grafts from C-H2bm12 (bm12, H2bm12) into C57BL/6 (C57BL/6, H2b) recipients immunosuppressed with a short course of anti-CD3 monoclonal antibodies to prevent acute rejection.

RESULTS

More than 80% of grafts survived for prolonged periods, but eventually all displayed macroscopic and microscopic evidence of chronic rejection. At histology, there was a progressive arterial intimal thickening as well as intense dermal fibrosis. This was accompanied by an inflammatory infiltrate consisting of lymphocytes and macrophages, but also of a considerable number of eosinophils. Mice with chronic rejection were unable to generate anti-donor MHC class II cytotoxic T lymphocyte activity at either 20 or 60 days after transplant. Furthermore, transplantation of bm12 skins on C57BL/6-congenic, Ig knock-out mice was associated with the development of a chronic rejection that was identical to that occurring in wild-type C57BL/6 animals, indicating that alloantibodies are not necessary in this model.

CONCLUSIONS

(1) Skin grafts may undergo chronic rejection with the characteristic lesions of vasculopathy and fibrosis; (2) chronic rejection of MHC class II-disparate skins may occur in the absence of direct cytotoxic T lymphocyte activity or alloantibodies.

Immunity to Asexual Blood Stages of Plasmodium: Is Resistance to Acute Malaria Adaptive or Innate?

Current models of immunity to blood stages of Plasmodium invoke a primary role for T-cell dependent processes and much recent evidence implicates Th1-type responses as crucial to the control of acute malaria. But do these data stand up to close scrutiny? Here, Andy Fell and Nick Smith review recent data from rodent and human studies and suggest that Th1-type responses may not after all be important in controlling malaria infection in the blood.

Fc Receptors Are Not Required for Antibody-Mediated Protection Against Lethal Malaria Challenge in a Mouse Model

The mechanisms by which Abs mediate protection during blood-stage malaria infections is controversial, with some evidence pointing to the direct effect of Abs on parasite invasion and growth, while other studies suggest that Abs act in cooperation with monocytes to achieve parasite inhibition. To determine whether the effector phase of protection in vivo to the rodent parasite Plasmodium yoelii yoelii requires Fc receptor bearing cells, we passively transferred immune sera into FcR γ-chain knockout mice. Inflammatory macrophages from these knockout mice were unable to mediate phagocytosis or Ab-dependent cell-mediated cytotoxicity (ADCC) through FcγRI, FcγRII, or FcγRIII. Passive transfer of either P. y. yoelii hyperimmune sera or anti-GST-PYC2 sera directed to the major merozoite surface protein (MSP-1) of this parasite enabled both BALB/cByJ mice and FcR γ-chain-deficient mice to resist lethal P. y. yoelii 17XL (Py17XL) challenge. mAb302, a protective IgG3 Ab, also passively protected both strains of mice. Most of these samples contain Ab isotypes that would not be able to protect mice if their protective effects required Ab-dependent cell-mediated cytotoxicity. These results establish that, in this infection, protection is directly mediated by Abs and does not require the participation of Fc receptors.

A Pathogenic Role of IL-12 in Blood-Stage Murine Malaria Lethal Strain Plasmodium berghei NK65 Infection

We studied whether the infection with a blood-stage murine malaria lethal Plasmodium berghei NK65 induces IL-12 production, and if so, how the IL-12 production is involved in the protection or pathogenesis. The infection of C57BL/6 mice enhanced mRNA expression of IL-12 p40 and also IFN-γ, IL-4, and IL-10 in both spleen and liver during the early course of the infection. It also enhanced the mRNA expression of TNF-α, Fas ligand, and cytokine-inducible nitric oxide synthase. Increased IL-12 p40 production was also observed in the culture supernatant of spleen cells and in sera of infected mice. In addition, the infection caused massive liver injury with elevated serum glutamic-oxaloacetic transaminase and serum glutamic-pyruvic transaminase activities and body weight loss. Treatment of these infected mice with neutralizing mAb against IL-12 prolonged the survival and diminished the liver injury with reduced elevation of serum serum glutamic-oxaloacetic transaminase and serum glutamic-pyruvic transaminase activities and decreased body weight loss. However, the anti-IL-12 treatment did not affect parasitemia, and all these mice eventually died. Similar results were obtained when infected mice were treated with neutralizing mAb against IFN-γ. Moreover, anti-IL-12 treatment greatly reduced the secretion and mRNA expression of IFN-γ in both spleen and liver. These results suggest that the lethal P. berghei NK65 infection induces IL-12 production and that the IL-12 is involved in the pathogenesis of liver injury via IFN-γ production rather than the protection.

Pathways and strategies for developing a malaria blood-stage vaccine

In the past 10 years, our knowledge of the malaria parasite has increased enormously: identification and analysis of parasite antigens, demonstration of protection of monkeys and mice following immunization with these antigens, and better understanding of the mechanisms of immunity to malaria and the pathogenesis of disease in malaria. Powerful new adjuvants have been developed, some of which--it is hoped--will be suitable for human use. Recently, a successful human trial of a vaccine aimed at sporozoites (the stage inoculated by mosquitoes) was completed. However, it is the red blood cell stage of the parasite that causes disease, and it is against this stage--in which the parasite grows at an exponential rate--that it has proven very difficult to induce a protective immune response by vaccination. This review focuses on recent exciting developments toward a blood-stage vaccine. We analyze the major obstacles to vaccine development and outline a strategy involving public- and industry-funded research that should result in development of a vaccine.

The spleen, IgG antibody subsets and immunity to Plasmodium berghei in rats

The development of IgG subclass-specific antibody responses to Plasmodium berghei in spleen-chimeric rats were monitored to determine if there was any relationship between IgG subset profiles and resistance. Strongly immune eusplenic rats respond to challenge with P. berghei by producing high levels of parasite-specific IgG2a, IgG2b and IgG2c but only modest levels of IgG1. Splenectomy profoundly affects the antibody response to infection. Thus, in splenectomized immunized rats, which harbour a chronic parasitaemia of 1%, the IgG2a, IgG2b and IgG2c responses peak 1 week later than in eusplenic immunized rats although the size of the peak is similar. More marked effects are apparent in the IgG1 response, the magnitude of which is far greater in splenectomized immunized rats than eusplenic immunized rats. Similar antibody profiles are seen in splenectomized immunized rats transplanted with a naive spleen. In contrast, splenectomized naive rats receiving either a transplant of a spleen from an immune rat or a transfer of immune spleen cells have high levels of IgG2a, IgG2b and IgG2c but modest levels of IgG1. However, only the former group of rats completely clears the parasite, the latter maintaining a chronic 1% parasitaemia. Thus, although complete resistance to P. berghei is always associated with high levels of parasite-specific IgG2a, IgG2b and IgG2c plus modest levels of IgG1, this is not a sufficient set of conditions to guarantee complete immunity. The IgG subset profile may be related to cytokine production; IFN-gamma was detected in the sera of rats receiving spleens from rats immune to P. berghei (modest IgG1 responses) but not in rats receiving spleens from naive animals (pronounced IgG1 responses).

Cytokines and antibody subclass associated with protective immunity against blood-stage malaria in mice vaccinated with the C terminus of merozoite surface protein 1 plus a novel adjuvant

A blood-stage malaria antigen comprising the C terminus of merozoite surface protein 1 fused to glutathione S-transferase, combined with an adjuvant formulation containing squalane, Tween 80, and pluronic L121 (AF), administered subcutaneously protected mice against death from a lethal Plasmodium yoelii infection. The protection induced by this antigen-adjuvant combination was compared with that induced by the antigen plus saponin in terms of survival from the lethal infection and clearance of parasitemia. The levels of gamma interferon and interleukin-4 in spleens were measured as indicators of Th1 and Th2 cell activation, and antibody classes and subclasses were determined by immunofluorescence. With a 10-micrograms dose of antigen and AF as adjuvant, all mice recovered, but with saponin as the adjuvant, there were only a few survivors. With 30 micrograms of antigen plus AF, the peak parasitemias were 10-fold lower than those with 10 micrograms; with saponin, survival was slightly improved. The levels of both gamma interferon and interleukin-4 rose more rapidly and to higher levels with AF as the adjuvant than with saponin, and the same was true for immunoglobulin G1 (IgG1), IgG2a, and IgG2b subclasses. Thus, in terms of both cytokine production and antibody levels, AF is a more potent adjuvant for a malaria vaccine than is saponin.